1. Field of the Invention
The present invention relates to a high pressure, RF-excited CO.sub.2 waveguide laser having an extended tuning range.
2. Description of Related Art
Wide-band tunable CO.sub.2 lasers have been needed for applications in laser communications, optical radar, pollution detection, and spectroscopy. The tuning range of a conventional low pressure CO.sub.2 laser is limited by the relatively small Doppler width of the CO.sub.2 laser transition. The development of the waveguide gas laser and its extension to high pressure sealed-off waveguide CO.sub.2 lasers has resulted in pressure broadening of the laser transition and larger available gain bandwidth for frequency tuning.
In an article by R. L. Abrams, "Gigahertz Tunable Waveguide CO.sub.2 Laser", Applied Physics Letters 25,304 (1974), a sealed off cw waveguide CO.sub.2 laser is described which has been continuously tuned over 1.2 GHz on a single transition of 10.6 .mu.m. The laser consists of a 9.5 cm.times.1.0 mm square discharge tube fabricated from polished BeO slabs. Line selection is achieved with a diffraction grating and tuning may be accomplished by piezoelectric control of the resonator length. It has been shown that, for a given resonator loss and output coupling, the gas pressure for optimum output power also results in maximum tunability. In order to achieve gigahertz tuning from a cw CO.sub.2 laser, under optimum conditions, smaller discharge diameters and correspondingly higher pressures are required. In the Abrams laser, a totally reflecting laser mirror is mounted on a bender bimorph and the opposite end of the waveguide laser is fitted with a 97% reflecting 150-lines/mm diffraction grating used in the Littrow configuration for line selection. Attempts to extend the technique to smaller discharge diameters and thus higher operating pressures and larger tuning ranges using a diffraction grating for line selection were unsuccessful.
Since it is known that RF-excited CO.sub.2 waveguide lasers can support a discharge at higher gas pressures than cw (dc) lasers and thus theoretically increase the tuning range above 1.2 GHz, prior attempts have been made to modify a RF-excited CO.sub.2 laser by replacing the laser transmitting mirror with a grating (for line selection) with the expectation that the tuning range could be increased above the 1.2 GHz provided by the cw laser. However, the results were similarly unsuccessful.
Certain applications require that the tuning range of CO.sub.2 waveguide lasers extend above 1.2 GHz. For example, satellites in low altitude orbits generally utilize optical sources, such as lasers, for communicating information between satellites and ground stations. These satellites require accommodation to be made for the Doppler frequency shift of .DELTA.V =V/.lambda., wherein V is the relative velocity between a transmitting and receiving satellite, and .lambda. is the wavelength of the transmitted optical light, at the receiver, as the satellites approach each other. A tuning range of at least 1500 MHz (.+-.750 MHz) would make possible the noted Doppler accommodation.
Thus, with the current limitation in tunability, a heterodyne receiver with complex signal processing to recover the message must be used on the receiver satellite to accommodate for the full Doppler shift in this application, increasing the cost and complexity of the overall system.
What is desired, therefore, is to provide a CO.sub.2 waveguide laser having a predetermined line and a tuning range encompassing 1500 MHz thus enabling the laser to be utilized with the satellite system previously described, (and in other applications requiring extended laser tuning ranges) this in turn allowing an inexpensive and less complex system for processing signal information to be utilized by the receiving satellite.